Numerous procedures involving catheters and other minimally invasive devices may be performed for a wide variety of treatments, such as ablation, angioplasty, dilation or other similar therapies. For example, there are many variations of cardiac arrhythmias with different causes, including atrial fibrillation, generally involving irregularities in the transmission of electrical impulses through the heart. To treat cardiac arrhythmias or irregular heartbeats, physicians often employ specialized ablation catheters to gain access to interior regions of a patient's body. Such catheters include tip electrodes or other ablating elements to create ablation lesions that physiologically alter the ablated tissue without removal thereof, disrupting or blocking electrical pathways through the targeted tissue. In the treatment of cardiac arrhythmias, a specific area of cardiac tissue, such as for example atrial rotors, having aberrant electrically conductive pathways with erratic electrical impulses is initially localized. A medical practitioner (such as a physician) may direct a catheter through a body passage including for example a blood vessel into the interior region of the heart that is to be treated. Subsequently, the ablating portion of the selected device is placed near the targeted cardiac tissue to be ablated, such as for example a pulmonary vein ostium or atrium.
An ablation procedure may involve creating one or more lesions in order to electrically isolate tissue believed to be the source of an arrhythmia. During the course of such a procedure, a physician may perform, for example, radio-frequency (RF) ablation which includes diagnosing aberrant tissue and destroying it with local administration of radio frequency energy. RF ablation may be performed by provide an RF electrical signal to one or more electrodes in contact with the tissue to be ablated, and the energy resistively heats the surrounding tissue. Eventually, the heating process destroys the selected cells surrounding the electrodes, and the ablation is completed.
Local conditions near the selected ablation site may change during delivery of RF ablation energy, for example due to fluid flow of blood and possibly saline solution. These fluids may be electrically conductive, and local fluid flow during ablation energy delivery to the electrodes may alter the electrodes' impedance. Depending upon the specific configuration of the medical system, the patient's anatomy and fluids near the ablation electrodes, the impedance characteristics during an ablation procedure may change by an amount ranging from a fraction of an ohm to more than 200 ohms.
This significant variation in local conditions may lead to consequences such as for example some undesirable tissue ablation, local generation of steam, or other overheating. Given variations in anatomy and the possibility of concurrent changes in the tissue treatment environment and the potential effect such variation may have on a therapeutic procedure, it is therefore desirable to provide a safe and effective medical ablation system having a feedback mechanism which automatically and continuously adjusts the ablation energy to ablate the desired tissue to be treated. It is also desirable to provide an ablation system and method for controlling the surface area and depth of ablation, and which automatically discontinues ablation once the desired treatment has been achieved.